It is known, as disclosed in U.S. Pat. Nos. 4,563,582 and 4,642,464 how one may provide a positron emission tomography (PET) scanner having a plurality of rows of scintillation crystals offset from each other on each photomultiplier tube. However, the length of the crystals in the axial direction was the same as the size of the photomultiplier tube in the axial direction. Because of this length restriction on the crystals, the axial resolution of the scanner was limited. In order to improve the axial resolution, a smaller crystal size in the axial direction is desirable. Of course, the same configuration shown in the above named patents could be used with smaller size photomultiplier tubes. However, this would greatly increase the cost and complexity of the scanner. As disclosed in U.S. Pat. No. 4,864,138, the axial dimension of the crystals could be reduced, yet the design employs irregular crystal shapes which taken in combination serve to mix the light between the set of photomultiplier tubes to provide crystal identification. However, the use of crystals of non-uniform shape and mass produces a system with inconsistent slice and angular sensitivities.
The present invention is directed to various improvements in a PET scanner by reducing the axial resolution using a uniform crystal size and shape, providing more measuring slices without adding any more photomultiplier tubes, reducing the size of the fundamental crystal identification area to that covered by two photomultiplier tubes, thereby enhancing the count rate performance of the system, and offsetting in the axial direction alternate modules in ring rather than offsetting alternate crystals within a module, in order to obtain axial sampling that is less than one-half the axial resolution.